Also called celluloids, they are plastic sheets used in traditional animation by hand-drawing and coloring images (or frames) on individual sheets, then shooting them in a sequence and then adding voices, music and SFX in post-production to create the effect of animation. This method of animation has been replaced with computer animation, so cel-animated shows are not widely seen anymore. They will contain punched holes to allow them to be stacked and/or aligned by means of a Peg Bar. The edges of a cell typically have a telltale unfinished look with rough brush strokes and discontinued lines, as the edges of the cels are meant to be cropped out of the final work.
For Danbooru, this tag refers to an individual frame of animation being scanned and uploaded.